1. Field of the Invention
The present invention relates to a method for the production of a film-foam laminate utilizing a heat welding technique. Preferably the lamination of the film to the foam is conducted in-line, i.e. immediately following extrusion of the thermoplastic foam and while such foam still contains a significant amount of residual blowing agent, laminating to said foam an oriented thermoplastic sheet. Advantage is taken of the plasticizing effect of the blowing agent upon the polystyrene foam whereby laminating temperatures requisite to achieve suitable film-foam bonding are substantially lowered, and hence disorientation of the oriented film by exposing it to the normally higher temperatures required for heat bonding such films to foam substrates is avoided.
2. Description of the Prior Art
U.S. Pat. Nos. 3,159,698 and 3,220,902 disclose methods for the production of laminations utilizing the heat of extrusion of either the foam material or the film material to achieve heat bonding as such materials exit from adjacent extruders. It will be obvious, however, that such techniques preclude prior biaxial orientation of the plastic film material, since in the disclosures of the aforenoted patents, such films are laminated as they are extruded and cast in place. In the method of the present invention, the preferred materials of laminate construction are oriented polystyrene film and polystyrene foam. In order to orient polystyrene film, it must be heated to above its glass transition temperature, stretched, and chilled while being held under tension. Thus, the methods disclosed in the aforenoted patents may not be employed to heat laminate oriented polystyrene film materials to foam substrates such as foamed polystyrene for example.
U.S. Pat. No. 2,917,217 discloses lamination of preformed sheet material to freshly extruded polystyrene foam. However, it is noted that this patent teaches the employment of adhesive materials to effect bonding of the polystyrene foam and sheet material, such as paper, being laminated thereto. Applicant, however, has found that the employment of extraneous adhesive materials to achieve a good laminar bonding of oriented polystyrene film to polystyrene foam may be eliminated by taking advantage of the softening effect certain blowing agents have on polystyrene foam whereby laminations of oriented styrene film thereto may be achieved at temperatures below those which cause disorientation of the polystyrene film. It will be obvious to those skilled in the art that disorientation of the polystyrene film would result in puckering and rupturing of the film surface during heat bonding of such film to the foam surface, as aforenoted, such disorientation occurring at above certain minimum temperatures. Additionally, disorientation of the film surface of the laminate results in a deterioration of physical properties of the final laminar structure including a decrease in tensile strength, ultimate elongation and toughness.